1. Field of the Invention
The subject invention generally relates to bandpass amplifier circuits and, in particular, to differential amplifier circuits having bandpass characteristics.
2. Description of Prior Art
Persons skilled in the art to which the invention relates are at least constructively aware of bandpass amplifiers such as disclosed in U.S. Pat. No. 2,510,868, issued to J. R. Day on June 6, 1950. In said amplifiers an emitter follower, having a first triode tube, is coupled by a ceramic crystal to the cathode of a second triode tube. With reference to the second tube, its cathode is coupled to ground by a resistor, its anode is coupled to a power supply by a load resistor and its grid is coupled to ground by a variable grid resistor. In addition, the grid is coupled by a variable capacitor to the output of the emitter follower. The variable capacitor and grid resistor are used to compensate for capacitance associated with a holder for the crystal.
Persons skilled in the art are also aware of solid state bandpass amplifiers. A typical amplifier of this type includes a transistor, a collector or load resistor, and emitter resistor and a voltage divider arrangement of resistors for biasing the transistor in its active region. A ceramic crystal connected in parallel with the emitter resistor gives the amplifier its bandpass characteristic.
The stability of transistor differential amplifier circuits has made them popular with integrated circuit designers and such amplifiers have been used in the IF sections of radios. In typical circuits, the bandwith and center frequency of the amplifiers is controlled with tuned collector circuits. The gain of differential amplifiers has been set, for example, by coupling the emitters of an amplifier with a resistor having a predetermined value.